The invention relates to an integrated acoustooptical component for frequency-shifting optical frequencies having at least one optical waveguide, and a sound generator which subjects the optical waveguide with acoustic waves which propagate in a longitudinal direction relative to the waveguide.
Such acoustooptical components for frequency-shifting or frequency displacement are required, for example, in optical heterodyne interferometers. Such an acoustooptical component, designed as a modulator, is known for example from Appl. Phys. Lett. 17, 265 (1970), Kuhn, Daks, Heidrich and Scott. In this known modulator, in the first instance a light guide is split, and each one of the two split light beams must then be broadened and fed to an optical system, which generates in each instance a broad parallel light beam. These broad light beams then pass through zones of acoustic waves, which propagate perpendicular to the light. They act in the manner of an optical grating of the Bragg type, so that the light passing through is diffracted. The first-order diffracted beam is evaluated and contains a frequency shift. An optical system effects a feedback into light guides; in this case, the first-order frequency-shifted beam is further evaluated. The disadvantage of this known arrangement resides in the fact that the broadening of the light beam and feedback into a light guide is very costly from the technological point of view. Since the acoustic wave acts only within the range of the width of the light beam, a high electrical power for the acoustic wave must be applied. Furthermore, the separating-off of the frequency-shifted beam is problematic from the technological point of view, since the angle of diffraction is relatively small.
Furthermore, Journal of Lightware Technology, Vol. 6, No. 6, June 1988, pages 903 ff, Hinkov, Opitz and Sohler, discloses an acoustooptical mode converter in which no broadening of the light guided in waveguides is required. The sound wave is guided parallel to the light guide; in this case, a birefringent material is required. To provide the frequency displacement, the waveguide is branched in a Y branching; in this case, the two arms of the Y branching are designed as TE-TM mode converters, which are operated with acoustic surface waves. The acoustic centre frequency, at which the mode conversion takes place for a specified optical wavelength, is to a large extent determined by the crystal birefringence. With the aid of a proton-exchanged layer, the birefringence in one of the arms is shifted. The acoustooptical interaction in the two arms then takes place at differing acoustic centre frequencies, so that the optical frequencies of the two emergent beams are shifted by the difference frequency. The disadvantage of this known arrangement resides in the fact that an expensive, birefringent material is necessary, such as for example LiNbO.sub.3. A further disadvantage is the relatively high temperature sensitivity of this process.
Moreover, Appl. Phys. Lett. 19 (1971) 428 ff, Kuhn, Heidrich and Lean, discloses an acoustooptical mode converter in which, again, no broadening of the light guided in waveguides is required. The sound wave is again guided parallel to the light guide in which at least two modes of the same polarisation must be capable of propagation. By scattering of the light at the acoustic wave, one mode is converted into another mode and in this case frequency-shifted. Disadvantages of this process reside in the very poor coupling of the modes, the poor separation of the light which is not frequency-shifted and that which is frequency-shifted and in the fact that the light guide must be at least bimode.